INTRODUCTION

Orthodontic wires, which generate the biomechanical forces communicated through brackets for tooth movement, are composed of four major base metal alloy types1,2: stainless steel (SS), cobalt-chromium-nickel (Co-Cr-Ni), nickel-titanium (Ni-Ti), and β-Ti. SS wires have excellent formability along with high mechanical properties (high strength and high modulus of elasticity).3–6 Co-Cr-Ni wires also have high elastic modulus and can easily be manipulated into desired shapes and then heat-treated to achieve considerable increases in strength and resilience.2,6 Both SS and Co-Cr-Ni wires can be soldered and welded for the fabrication of complex appliances. Ni-Ti wires, which contain approximately equiatomic proportions of Ni and Ti, offer a modulus of elasticity that is nearly one-fifth that of the SS wires, along with very wide elastic working range.1,2 The Ni-Ti wires possess superelasticity and shape-memory effect.7,8 The β-Ti wires were conceived for orthodontic use about three decades ago and offer a modulus of elasticity measuring approximately half that of the SS wires.3–5 Because the β-Ti wires offer good mechanical properties, namely, excellent formability and excellent corrosion resistance (Ni-free), they are widely used, especially for patients in orthodontics with an allergy to Ni. However, these Ti alloy (Ni-Ti and β-Ti) wires are difficult to join by soldering and welding as a result of the high reactivity of Ti with gasses such as oxygen and nitrogen at high temperatures.9,10 Therefore, soldering and welding of Ti and its alloys can only be performed under vacuum or argon atmosphere.11,12 However, there are several research reports13–16 about the welding of Ti alloy wires using conventional dental welding. These studies imply the possibility of welding for Ti alloy orthodontic wires.

One of the methods of joining Ti and its alloys is laser welding under argon shielding.12 Laser welding is a convenient method with which to connect any type of dental alloys, especially Ti, because Ti has higher laser beam absorption and lower thermal conductivity (high heat absorption in small area) than do the other dental alloys.17 The advantages of laser welding include the following: there are fewer heat effects on the surrounding area to be welded, and no materials (such as investment materials, furnace, and gas torch) are necessary for conventional dental soldering.18 Another advantage of laser welding is that it allows welding of different types of dental alloys regardless of the different melting points for the alloys.19,20 In general, alloys with similar compositional elements could produce favorable results for the welding of different alloys because of the similarity of their melting points. Although the use of laser welding has become widespread in prosthodontics, there are few reports21–23 regarding the laser welding of orthodontic wires. Furthermore, no other investigation has been performed regarding the laser welding of different types of orthodontic wires, as stated above.

The significance of laser welding of orthodontic wires is that the force and torque applied for tooth movement can be controlled by welding different sizes, shapes (square, rectangular, round, and twist), and types of orthodontic wires. Note that prefabricated combination archwires with a rectangular anterior segment and a round posterior segment have recently become commercially available for Ni-Ti, β-Ti, and SS used in clinical orthodontics. Therefore, the objective of this study is to investigate the deflection load characteristics of homogeneous and heterogeneous joints made by laser welding using four types of orthodontic wires.

MATERIALS AND METHODS

Four types of rectangular wire (0.017 inch × 0.025 inch) were used: SS (STSS1725, (G&H Wire Company, Greenwood, Ind); Co-Cr-Ni (Colboloy, STRCB1725, G&H Wire Company); β-Ti (BT3, SBT3-1725, G&H Wire Company); and Ni-Ti (SENT1725, G&H Wire Company). The elemental compositions of the wires are shown in Table 1.

Table 1 Elemental Composition (wt.%) for the Wires^a

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Specimen Preparation for Three-Point Bending Test

The 12-mm–long wires were cut from each type of straight wire using a low-speed diamond saw (Isomet, Buehler, Lake Bluff, Ill) with water cooling. After the cut surfaces were polished with silicon-carbide paper (No. 400), cut surfaces of two wires were abutted against each other and fixed in a custom-made jig (Figure 1). They were then laser welded using a dental laser-welding machine (Neo-Laser L, Girrbach Co, Pforzheim, Germany) under argon gas shielding.

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Homogeneous end-to-end joint specimens were prepared for each type of wire, and heterogeneous end-to-end joint specimens were made in combination with four types of wires. Two types of welding methods were used: two-point welding (one point on each 0.025-inch surface) and four-point welding (two points were added on each 0.017-inch surface). The pulse duration and output voltage for laser welding were fixed at 5 ms and 150 V, respectively. The spot diameters used were 0.7 mm for the 0.025-inch surface and 0.5 mm for the 0.017-inch surface, respectively. Nonwelded wires (24 mm long) were also prepared as controls.

RESULTS

The load/deflection curves of the control specimens are presented in Figure 2. The highest maximum deflection was obtained for SS wire (21.7 ± 0.8 N), followed by Co-Cr-Ni (20.0 ± 0.3 N), β-Ti (13.9 ± 1.3 N), and Ni-Ti (6.6 ± 0.4 N) wires. The curve of Ni-Ti showed superelasticity (the curve returned to the starting zero point).

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The deflection loads for two-point welding and four-point welding are presented in Table 2 (homogeneously welded combinations) and Table 3 (heterogeneously welded combinations) and depicted in Figure 3 (two-point welding) and Figure 4 (four-point welding), respectively. All of the homogeneously welded wires showed lower deflection loads compared to the corresponding control wires and exhibited higher deflection loads compared to the heterogeneously welded combinations. In the homogeneous combinations, Co-Cr-Ni/Co-Cr-Ni showed a significantly (P < .05) higher deflection load than did the remaining homogeneously welded groups. Most of the homogeneously welded specimens deflected to the maximum deflection (3 mm), except for the two-point welding of SS, Co-Cr-Ni, and Ni-Ti, in which a few specimens fractured right before the maximum deflection of 3 mm. None of the β-Ti/β-Ti specimens for both two-point and four-point welding methods fractured before the maximum deflection. Figure 5 shows the load/deflection curves of homogeneously welded wires (four-point welding). The highest deflection load of the homogeneously welded specimens was obtained for Co-Cr-Ni. The maximum deflection loads of SS/SS dropped down and reached a level similar to that of β-Ti/β-Ti. In heterogeneous combinations, SS/Co-Cr-Ni and β-Ti/Ni-Ti showed higher deflection loads than did the remaining heterogeneously welded combinations (significantly higher for SS/Co-Cr-Ni). Most of the heterogeneously welded specimens fractured during the three-point testing, except for the SS/Co-Cr-Ni combinations, in which most of the specimens deflected at the maximum deflection.

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Table 2 Deflection Loads (N) of Homogeneously Welded Combinations^a

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Table 3 Deflection Loads (N) of Heterogeneously Welded Combinations^a

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Significance (P < .01) was found for the interaction between the two factors (materials combination and welding method) in both of the homogeneous and heterogeneous combinations. However, no significant difference in deflection load was found between four-point and two-point welding in either homogeneous or heterogeneous combinations, even though the four-point welding group showed a slightly higher deflection load than did the corresponding two-point welding group in most groups.

DISCUSSION

All homogeneous welded groups except for the Ni-Ti/Ni-Ti group showed highly reduced deflection loads compared to the corresponding control group. The highest reduction was observed for SS/SS (63% [two-point] to 67% [four-point] reduction), followed by Co-Cr-Ni/Co-Cr-Ni (35% [four-point] to 44% [two-point] reduction), β-Ti/β-Ti (35% [four-point] to 42% [two-point] reduction), and Ni-Ti/Ni-Ti (12% [four-point] to 17% [two-point] reduction).

When SS/SS and Co-Cr-Ni/Co-Cr-Ni are compared, SS and Co-Cr-Ni possess similar compositional elements (Table 1), except for iron (Fe) and Co. The SS is composed of 69.5% Fe with no Co, whereas the balance of Fe is 22.9% for the Co-Cr-Ni, which contains approximately 40% Co. In addition, the Co-Cr-Ni contains 7.1% molybdenum (Mo), which is not contained in the SS. Although Co and Fe have similar properties (thermal conductivity, rate of laser beam absorption, melting point, etc) that are different from those of Mo, the difference in composition for Fe, Co, and/or Mo could be the reason for the highly reduced deflection load for SS/SS vs that seen for Co-Cr-Ni/Co-Cr-Ni. In homogeneously welded Ti-based wires (β-Ti and Ni-Ti), the Ni-Ti/Ni-Ti showed a lesser reduction of deflection load than did the β-Ti. These results could be explained by the fact that the Ni-Ti wire exhibited the lowest value of elastic modulus and deflection load among the materials tested. If the Ni-Ti/Ni-Ti specimens deflected to the maximum deflection (3 mm), the maximum deflection loads of the Ni-Ti/Ni-Ti specimens will be similar to those of the control Ni-Ti, since the control Ni-Ti also exhibits the lowest elastic modulus and maximum deflection load among the controls (Figure 2). On the other hand, the proportional limit (maximum stress at elastic limit) of the β-Ti/β-Ti specimens is lowered by laser welding, therefore resulting in the reduced deflection load (35–42% reduction).

Heterogeneously welded combinations showed lower deflection loads compared to the homogeneously welded wires. In heterogeneously welded groups, SS/Co-Cr-Ni and β-Ti/Ni-Ti showed higher deflection loads than did the remaining heterogeneously welded combinations (significantly higher for SS/Co-Cr-Ni). These results indicate that the Ti-based wires do not have good affinity with SS and Co-Cr alloys. Conversely, SS vs Co-Cr-Ni and β-Ti vs Ni-Ti have an affinity for laser welding, which can possibly be attributed to the similarity of their mechanical properties or compositional elements.

When each of the Co-Cr-Ni and β-Ti wires was welded homogeneously and SS/Co-Cr-Ni was welded heterogeneously by four-point welding, they had less reduced deflection than did the corresponding combinations joined by two-point welding. These results indicate that the homogeneously welded Co-Cr-Ni and β-Ti and heterogeneously welded SS/Co-Cr-Ni might show more improved deflection loads using a two-point welding if higher laser conditions (high laser voltage and high pulse duration) were used. Regarding the correlation between the SS and the Co-Cr-Ni, the homogeneously welded SS using two-point welding showed slightly higher deflection load than did the heterogeneously welded SS/Co-Cr-Ni. However, using four-point welding, the deflection load of the heterogeneously welded SS/Co-Cr-Ni was higher than that of the homogeneously welded SS (in the middle between the homogeneously welded SS and the homogeneously welded Co-Cr-Ni). This is because the homogeneous four-point–welded SS had a more reduced deflection load compared to the two-point–welded SS. In addition, the heterogeneous four-point–welded SS/Co-Cr-Ni showed an improved deflection load compared with the two-point–welded SS/Co-Cr-Ni. These results indicate that the heterogeneously welded SS/Co-Cr-Ni might be affected by the welding method (four-point > two-point) in conjunction with the favorable affinity between the SS and the Co-Cr-Ni for laser welding.

CONCLUSIONS

• Homogeneously laser-welded Ni-Ti showed the lowest reduction of deflection load among the wires tested.

• The heterogeneously welded combination showed lower deflection loads compared to those of homogeneously welded wires, except in the case of the SS/Co-Cr-Ni combination.

• The Ti-based orthodontic wires (β-Ti and Ni-Ti) do not have good affinity with SS and Co-Cr wires in laser welding.